The present invention is directed to a display device for representing bright images on a dark background. The display device comprises a light valve and in particular a liquid crystal cell which has a medium that can be switched zone-wise between at least two optical states, means for selectively switching the zones of the medium and a light trap body, which consists of a material having an index of refraction greater than 1, contains fluorescent particles and has a light outlet window assigned to each of the switchable zones of the medium to pass light therethrough.
A display in which a liquid crystal cell having a medium which is arranged with a light trap consisting of a plate containing fluorescent material and outlet windows is disclosed in the co-pending U.S. patent application Ser. No. 747,035which issued as U.S. Pat. No. 4,142,781 and includes a disclosure of German patent applications P 25 54 226 and P 26 16 669. The devices disclosed in the above U.S. application have a high contrast in the representations of the cell due to the light trap or fluorescent body intercepting a large part of the surrounding light striking it by means of fluorescent scattering and total reflection and then emitting the light from the body at a relatively high intensity at special outlet or emergent windows. The brightness at the window is dependent on the ratio between the light collecting area to the light emitting surface area of the windows. Thus, the greater the ratio or brightness amplification factor, the brighter the image at the windows.
If the light valve allows passage of the excitation light for the fluorescent particles when the medium is in a rest condition or state, a particularly large collector surface can be achieved. For this purpose, it is contemplated to either develop polarizers, which are permeable to the excitation light (see U.S. Ser. No. 747,035) or cover only the switchable portions of the display element with the polarizer as proposed in U.S. patent application Ser. No. 791,406, which is now abandoned and includes the disclosure of German patent application P 26 19 286. In the case of a light valve using a liquid crystal, it has been proposed to orientate the liquid crystal layer outside the display elements in such a manner that the cell becomes transparent in those zones (see German application No. P 26 25 864).